This invention pertains to a dampening roller assembly for delivering water to a printing plate of an offset printing press.
The offset printing process requires roller trains to deliver ink and water to printing plates. The ink and water can be delivered through the same roller train or through separate roller trains. If the water is delivered through its own independent roller train, it is called dampening roller train. The dampening roller train normally consists of several rollers, including a dampening form roller, dampening chrome roller, and possibly one or more dampening rider rollers. The form roller and rider roller are often friction driven which provide a "squeeze" operation to smooth out the water laydown and deliver even water film. The chrome roller is gear-driven and plated with oilphobic material (such as chrome) on the roller surface, which repels the ink and adheres to the water. The combination and total number of dampening rollers depend on the type of dampening delivery system and targeted printing quality. Two-roller, three-roller or four-roller trains arranged linearly in series are employed in conventional dampening delivery systems.
When pursuing higher printing quality and better water laydown uniformity, the utilization of more dampening rollers, to provide more nips, is often performed. However, the existing serial roller layout requires a significant amount of space, many gears and drive machinery for the driven rollers, and relies heavily on the covering condition for each individual roller and the nip setting in between rollers.
Currently there are three types of serial dampening roller layouts: two-roller trains, three-roller trains and four-roller trains. The two-roller serial train has one rubber dampening form roller and one chrome roller, and is widely used for low speed and normal printing quality applications. The dampening water is applied to a driven chrome roller first and is transferred through a form roller to reach the plate cylinder. There are only two nips involved in the water transferring and smoothing process for the serial dampening train of rollers.
The three-roller serial train provides one rubber dampening form roller, one driven chrome roller and one rubber dampening rider roller, and is used for medium-to-high speed and high print quality applications. The dampening water is applied to the rider roller first and is transferred through a chrome roller and a form roller to reach the plate cylinder. There are three nips involved in the water transferring and smoothing process for the serial dampening train of rollers.
The four-roller serial dampening train has one rubber dampening form roller, one driven chrome roller, one rubber dampening transfer roller and another driven chrome roller, and is used for high speed and high print quality applications. The supplied dampening water is applied to a driven chrome roller first and is then transferred through a transfer roller, another driven chrome roller and one water form roller to finally reach the plate cylinder. There are four nips involved in the water transferring and smoothing process in the four-roller serial dampening train. Due to the serial layout of the four-roller train, the physical parameters for each contact (i.e. nip setting, rubber material and durometer) between rollers become very crucial to the water transfer. Slight variances in the nip settings, rubber material or the durometer for the serially arranged rollers can be magnified significantly thereby potentially reducing print quality due to the serial arrangement of the rollers. Furthermore, in the four roller train, both of the driven chrome rollers are required to be gear-driven to prevent possible slippage which renders the gear train design more complicated and requires a significant amount of space to be dedicated for the motor and gear systems in the four-roller arrangement.